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Biomedical subjects

R J Ordidge

Publications and source records attributed to R J Ordidge.

At least 19 recordsLinked to original sources
PubMed2001

Simultaneous noninvasive measurement of CBF and CBV using double-echo FAIR (DEFAIR).

A new method for measuring cerebral blood flow (CBF) and cerebral blood volume (CBV) noninvasively using MRI is presented. The approach is based on the technique of arterial spin labelling (ASL), in which CBF-based contrast is generated by controlled modulation of the longitudinal magnetization of the blood. The proposed method also uses differences in T(2) between tissue and blood to differentiate the two compartments and allow assessment of the relative size of each. Two successive EPI images are acquired following spin preparation using either a slice-selective or global inversion pulse, and the technique is therefore referred to as double-echo FAIR (DEFAIR). DEFAIR is demonstrated in the normal gerbil brain and during hypothermia, where reductions of both CBF and CBV are known to occur. It is also shown theoretically that this method can be extended to include a measurement of oxygen extraction fraction. The main drawbacks of the technique are the long acquisition time and relatively low sensitivity to hemodynamic changes compared to conventional qualitative T2(*)-weighted BOLD contrast, which may limit its applicability and practical use in monitoring functional cerebral activation. However, the technique can be used repetitively in longer-term time course studies due to its noninvasive and quantitative nature.

Animals↗
PubMed2000

Acute changes in MRI diffusion, perfusion, T(1), and T(2) in a rat model of oligemia produced by partial occlusion of the middle cerebral artery.

Oligemic regions, in which the cerebral blood flow is reduced without impaired energy metabolism, have the potential to evolve toward infarction and remain a target for therapy. The aim of this study was to investigate this oligemic region using various MRI parameters in a rat model of focal oligemia. This model has been designed specifically for remote-controlled occlusion from outside an MRI scanner. Wistar rats underwent remote partial MCAO using an undersize 0.2 mm nylon monofilament with a bullet-shaped tip. Cerebral blood flow (CBF(ASL)), using an arterial spin labeling technique, the apparent diffusion coefficient of water (ADC), and the relaxation times T(1) and T(2) were acquired using an 8.5 T vertical magnet. Following occlusion there was a decrease in CBF(ASL) to 35 +/- 5% of baseline throughout the middle cerebral artery territory. During the entire period of the study there were no observed changes in the ADC. On occlusion, T(2) rapidly decreased in both cortex and basal ganglia and then normalized to the preocclusion values. T(1) values rapidly increased (within approximately 7 min) on occlusion. In conclusion, this study demonstrates the feasibility of partially occluding the middle cerebral artery to produce a large area of oligemia within the MRI scanner. In this region of oligemic flow we detect a rapid increase in T(1) and decrease in T(2). These changes occur before the onset of vasogenic edema. We attribute the acute change in T(2) to increased amounts of deoxyhemoglobin; the mechanisms underlying the change in T(1) require further investigation.

Animals↗
PubMed2000

The measurement of diffusion and perfusion in biological systems using magnetic resonance imaging.

The aim of this review is to describe two recent developments in the use of magnetic resonance imaging (MRI) in the study of biological systems: diffusion and perfusion MRI. Diffusion MRI measures the molecular mobility of water in tissue, while perfusion MRI measures the rate at which blood is delivered to tissue. Therefore, both these techniques measure quantities which have direct physiological relevance. It is shown that diffusion in biological systems is a complex phenomenon, influenced directly by tissue microstructure, and that its measurement can provide a large amount of information about the organization of this structure in normal and diseased tissue. Perfusion reflects the delivery of essential nutrients to tissue, and so is directly related to its status. The concepts behind the techniques are explained, and the theoretical models that are used to convert MRI data to quantitative physical parameters are outlined. Examples of current applications of diffusion and perfusion MRI are given. In particular, the use of the techniques to study the pathophysiology of cerebral ischaemia/stroke is described. It is hoped that the biophysical insights provided by this approach will help to define the mechanisms of cell damage and allow evaluation of therapies aimed at reducing this damage.

Animals↗
PubMed1999

Rapid T2* mapping using interleaved echo planar imaging.

Magnetic resonance imaging methods that are sensitive to T2* are widely used in the study of blood oxygenation changes, most notably in functional studies of the brain. In these studies the signal intensity change in T2*-weighted imaging is related to the coupling of cerebral blood flow and metabolism. Rapid measurement of T2* itself would offer a valuable method to quantify blood oxygenation changes indirectly and monitor their time course. An interleaved echoplanar imaging (EPI) sequence is presented here that allows maps of T2* to be generated in a few seconds. The sequence benefits from reduced geometric distortion and an improved point spread function compared with single-shot EPI. A comparison among a set of T2*-weighted interleaved EPI images, single-shot EPI, and conventional gradient-echo and spin-echo methods is made using a compartmentalized doped water phantom. The interleaved sequence yields accurate T2* values when compared with reference measurements made using the slower gradient-echo technique. Data acquired from the rat brain at 2.35 T prior to and during an anoxic challenge show, with high temporal resolution, the reduction in T2* associated with increased levels of deoxyhemoglobin.

Animals↗
PubMed1999

Early changes in water diffusion, perfusion, T1, and T2 during focal cerebral ischemia in the rat studied at 8.5 T.

The time evolution of water diffusion, perfusion, T1, and T2 is investigated at high magnetic field (8.5 T) following permanent middle cerebral artery occlusion in the rat. Cerebral blood flow maps were obtained using arterial spin tagging. Although the quantitative perfusion measurements in ischemic tissue still pose difficulties, the combined perfusion and diffusion data nevertheless distinguish between a "moderately affected area," with reduced perfusion but normal diffusion; and a "severely affected area," in which both perfusion and diffusion are significantly reduced. Two novel magnetic resonance imaging observations are reported, namely, a decrease in T2 and an increase in T1, both within the first few minutes of ischemia. The rapid initial decrease in T2 is believed to be associated with an increase in deoxyhemoglobin levels, while the initial increase in T1 may be related to several factors, such as flow effects, an alteration in tissue oxygenation, and changes in water environment.

Animals↗
PubMed1999

Implementation of quantitative FAIR perfusion imaging with a short repetition time in time-course studies.

Flow-sensitive alternating inversion recovery (FAIR) is a pulsed arterial spin labeling magnetic resonance imaging method for perfusion quantification. In its standard implementation for quantification with full longitudinal relaxation between acquisitions, its use in time-course investigations of rapidly changing flow values is limited. The time efficiency can be improved by decreasing the repetition time but quantification becomes problematic. This situation is further complicated if a whole-body radiofrequency transmit coil is not used since fresh blood spins will flow in from outside the coil. To alleviate these problems, the use of global pre-saturation is proposed. The resulting expression for the flow signal depends on the relationship between the imaging parameters and the coil inflow time and can be significantly simplified under certain combinations of these parameters. With this implementation of FAIR, quantitative flow maps of gerbil brains were obtained with a 3 minute time resolution in a study of the effects of reperfusion. The pre-occlusion flow measurements were in good agreement with values obtained by the standard FAIR implementation and by other techniques, but the low values following occlusion were underestimated due to the increased transit times.

Animals↗
PubMed1999

Cerebral tissue water spin-spin relaxation times in human neonates at 2.4 tesla: methodology and the effects of maturation.

Using a 4-echo spin-echo sequence, cerebral T2 was measured in specific anatomic regions in eleven healthy newborn infants, whose gestational plus postnatal ages (GPAs) lay between 37 and 42 weeks. For a region in the pons, T2 was 141+/-9 ms (mean +/- standard deviation), and no significant dependence upon GPA was seen. In the thalamus mean T2 was 136+/-13 ms, and T2 demonstrated a significant negative linear dependence upon age (r = 0.690; p < 0.02). In periventricular and frontal regions, mean T2 were 217+/-33, and 228+/-32 ms respectively, and more marked negative linear correlations with age were observed (r = 0.833; p < 0.001 and r = 0.722; p < 0.02). For these regions, the rate of T2 decrease with age appeared to be related to known patterns of myelination. For the parietal region studied, mean T2 was 204+/-34 ms, no significant dependence upon GPA being seen. T2 shows promise as an objective measure of cerebral development in the perinatal period.

Brain↗
PubMed1999

Reperfusion in a gerbil model of forebrain ischemia using serial magnetic resonance FAIR perfusion imaging.

BACKGROUND AND PURPOSE: Existing methods for the quantitative measurement of the changing cerebral blood flow (CBF) during reperfusion suffer from poor spatial or temporal resolution. The aim of this study was to implement a recently developed MRI technique for quantitative perfusion imaging in a gerbil model of reperfusion. Flow-sensitive alternating inversion recovery (FAIR) is a noninvasive procedure that uses blood water as an endogenous tracer. METHODS: Bilateral forebrain ischemia of 4 minutes' duration was induced in gerbils (n=8). A modified version of FAIR with improved time efficiency was used to provide CBF maps with a time resolution of 2.8 minutes after recirculation had been initiated. Quantitative diffusion imaging was also performed at intervals during the reperfusion period. RESULTS: On initiating recirculation after the transient period of ischemia, the FAIR measurements demonstrated either a symmetrical, bilateral pattern of flow impairment (n=4) or an immediate side-to-side difference that became apparent with respect to the cerebral hemispheres in the imaged slice (n=4). The flow in each hemisphere displayed a pattern of recovery close to the preocclusion level or, alternatively, returned to a lower level before displaying a delayed hypoperfusion and a subsequent slow recovery. The diffusion measurements during this latter response suggested the development of cell swelling during the reperfusion phase in the striatum. CONCLUSIONS: The CBF during the reperfusion period was monitored with a high time resolution, noninvasive method. This study demonstrates the utility of MRI techniques in following blood flow changes and their pathophysiological consequences.

Animals↗
PubMed1998

MRI measurements of cerebral deoxyhaemoglobin concentration [dHb]--correlation with near infrared spectroscopy (NIRS).

Changes in physiological parameters such as cerebral blood flow, cerebral blood volume, oxygen extraction, and the size and distribution of cerebral blood vessels, result in changes in the local concentration of deoxyhaemoglobin ([dHb]). The purpose of this study was to quantitatively investigate the dependence of the R2* relaxation rate upon the [dHb] per voxel. Five neonatal piglets were studied in a 7 T/20 cm bore magnet. MRI was conducted using a 2.5 cm diameter surface coil placed over the parietal lobes. Four progressively T2*-weighted images were acquired, allowing the absolute quantitation of R2*. Simultaneous near infrared spectroscopy (NIRS) measurements were made from an area encompassing the MR imaging slice, and allowed the absolute quantitation of [dHb]. The arterial oxygen saturation (SaO2) of the piglet was lowered stepwise by decreasing the fractional inspired oxygen concentration (FiO2), which precipitated a change in [dHb]. NIRS and MRI measurements were made at each FiO2 step. The results demonstrate an extremely strong, linear relationship between R2* as determined by MRI and [dHb], as measured by NIRS. Whereas NIRS can only give us a global measure of [dHb], the results suggest the future use of MRI in producing high resolution relaxation rate maps related to the [dHb] distribution of the brain.

Animals↗
PubMed1998

Temporal and anatomical variations of brain water apparent diffusion coefficient in perinatal cerebral hypoxic-ischemic injury: relationships to cerebral energy metabolism.

Cerebral apparent diffusion coefficients (ADCs) were determined in nine newborn piglets before and for 48 h after transient hypoxia-ischemia. Phosphorus MRS revealed severely reduced cerebral energy metabolism during the insult and an apparently complete recovery 2 h after resuscitation commenced. At this time, mean ADC over the imaging slice (ADCglobal) was 0.88 (0.04) x 10(-9) m2 x s(-1) (mean (SD)), which was close to the baseline value of 0.92 (0.4) x 10(-9) m2 x s(-1). In seven of the animals, a "secondary" failure of energy metabolism then evolved, accompanied by a decline in ADCglobal to 0.64 (0.17) x 10(-9) m2 x s(-1) at 46 h postresuscitation (P < 0.001 versus baseline). For these seven animals, ADCglobal correlated linearly with the concentration ratio [phosphocreatine (PCr)]/[inorganic phosphate (Pi)] (0.94 < r < 0.99; P < 0.001). A nonlinear relationship was demonstrated between ADCglobal and the concentration ratio [nucleotide triphosphate (NTP)]/[Pi + PCr + 3 NTP]. The ADC reduction commenced in the parasagittal cortex before spreading in a characteristic pattern throughout the brain. ADC seems to be closely related to cerebral energy status and shows considerable potential for the assessment of hypoxic-ischemic injury in the newborn brain.

Animals↗
PubMed1998

A quantitative method for fast diffusion imaging using magnetization-prepared TurboFLASH.

For the in vivo measurement of the apparent diffusion coefficient (ADC), it is desirable for the total imaging time to be as short as possible. One technique is based on a TurboFLASH acquisition in which the diffusion gradients are inserted into a driven equilibrium Fourier transform (DEFT) combination of hard pulses. However, this sequence has the disadvantage that eddy current-induced inhomogeneities lead to incomplete refocusing of the magnetization during the diffusion preparation and to incorrect ADC values. A modification to the sequence is suggested that eliminates this error by phase-cycling the second 90 degrees pulse of the preparation. This study also investigates the effect of a reduced delay time between acquisitions on the accuracy of the measurement. The quality of the TurboFLASH sequence is demonstrated by experimental validation on an agar phantom and in vivo on the rat brain using a high-field (8.5 T) system. Reduction of the interexperiment delay time is shown to be achievable to a certain degree without compromising the measurement accuracy.

Animals↗
PubMed1997

Anisotropic water diffusion in white and gray matter of the neonatal piglet brain before and after transient hypoxia-ischaemia.

Measurements of tissue water apparent diffusion coefficient (ADC) performed with diffusion sensitization applied separately along the x, y, and z axes revealed significant diffusion anisotropy in both cerebral white and gray matter in six newborn (< 24 h old) piglets. Mean baseline white matter ADC for a particular region of interest was 125.8% (SD 32.0%; p < .001) greater when the diffusion gradients were applied along the y axis as compared to along the x. For the cortical gray matter region considered, the situation was reversed, the mean ADC value measured along x exceeding that along y by 15.2% (SD 6.1%; p < .01). Forty-three hours subsequent to a transient cerebral hypoxic-ischaemic insult, phosphorous MRS measurements indicated that the animals had suffered severe secondary cerebral energy failure. This was accompanied by a significant (p < .01) decrease in the white matter anisotropy, such that the mean y direction ADC now exceeded that along the x by only 70.9% (SD 29.4%; p < .03). There was no change in the gray matter anisotropy. The average of the ADC values measured in the x, y, and z directions had decreased by 35.3% (SD 18.5%; p < .01) in white matter and 31.4% (SD 21.9%; p < .05) in cortical gray matter. Diffusion anisotropy measurements may provide additional information useful in the characterisation of hypoxic-ischaemic injury in the neonatal brain, and must be considered if tissue water ADC values are to be unambiguously interpreted in this context.

Animals↗
PubMed1996

Frequency offset corrected inversion (FOCI) pulses for use in localized spectroscopy.

Gradient localized spectroscopy techniques suffer from a well documented spatial localization error caused by the difference in chemical shifts between resonances. This results in the acquisition of spectra from partially overlapping spatial regions of the sample, with each resonance representing a different region. The image-selected in vivo spectroscopy technique uses hyperbolic secant inversion pulses, where the main limitation in reducing this error is in the RF power available for application of the selective RF pulse. This spatial localization error may be dramatically reduced by increasing, and temporally shaping, the gradient pulse during slice-selective spin inversion. The performance of these RF pulses have been experimentally verified.

Magnetic Resonance Spectroscopy↗
PubMed1995

Preliminary observations of transverse relaxation rates obtained at 3 tesla from the substantia nigra of adult normal human brain.

The transverse relaxation rates R2 and R2* were measured at 3 T in the substantia nigra of the midbrain of adult normal human controls. The relaxation rate arising from magnetic inhomogeneities, R2', was calculated from the relationship R2* = R2 + R2'. No significant differences were found for any parameters between left and right side substantia nigra regions, however, a significant inverse correlation was found between R2 and R2' (r = 0.70, p = 0.035). Water diffusion, in the presence of paramagnetic metal ions, may play a role in determining the relative contributions to reversible and irreversible transverse relaxation and may explain the inverse correlation demonstrated here.

Aged↗
PubMed1995

Increased iron-related MRI contrast in the substantia nigra in Parkinson's disease.

Elevated iron levels in the substantia nigra (SN) of the brain in Parkinson's disease (PD) may mediate lipid peroxidative reactions, promoting SN neuronal death. To assess SN iron accumulation in living PD patients and its relation to motor performance, we measured, in 13 nondemented PD patients and 10 normal control subjects, simple reaction time (SRT) and simple movement time (SMT), followed by head MRI in a 3-tesla system. We measured T2 and T2* in the right and left SN of all subjects and calculated R2', the relaxation rate due to local magnetic field in-homogeneities, from these values. Asymmetries of 1/T2 (R2), 1/T2* (R2*), or R2' versus asymmetries of SRT and SMT were assessed in eight PD subjects who had not taken anti-PD medication(s) for 12 hours. The average of right and left SN values for R2 was lower, and R2* and R2' were higher, in PD patients than in controls (R2, p = 0.046; R2*, p = 0.001; R2', p < 0.001). R2' best predicted group differences. The asymmetry of SRT performance was highly correlated with asymmetries of SN R2* (0.91; p = 0.001) and R2' (0.72; p = 0.03). These results strongly suggest that the increases in iron levels seen postmortem in the SN in PD are reflected in increased iron-related MRI contrast at 3 tesla in living PD patients. Correlations with motor performance in PD suggest that the clinical severity of PD may be related to SN iron accumulation.

Adult↗